Will much of New Orleans be underwater by 2100?

At a location in Mississippi, water from the Mississippi River, left, stands at 95.7 feet above sea level. A rise in sea level is one reason that researchers are concerned about the Mississippi River Delta.

Katie Carter/The Vicksburg Evening Post/AP

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Unless enormous amounts of soil are dumped onto the Mississippi River Delta, the region could lose up to 5,212 square miles of land to ocean and tidal marsh by 2100 – a result of sea-level rise and the land sinking.

Even if levees are intentionally breached to supply fresh sediment to the delta, the Mississippi River would fall billions of tons short of delivering enough silt to maintain a delta that looks anything like it does today.

The boundary conditions appear unforgiving. For instance, all that remains of New Orleans would probably be the French Quarter and the airport. Lake Pontchartrain would lie beneath a vast bay. Along its southernmost reaches, the Mississippi River would remain a river only by virtue of the levees raised to contain it.

The researchers acknowledge that the study is a first cut at putting numbers to the problem. Others are likely to devise more precise estimates. “But even if we’re off by 50 percent, it’s still bad,” says Michael Blum, Dr. Roberts’s colleague on the work.

During the past 12,000 years, according to the study, up to 3.5 trillion tons of sediment have been deposited into a valley stretching from Memphis, Tenn., to the Gulf of Mexico. Some 80 percent of that muck makes up the delta plain.

On average, the river delivered 400 million to 500 million tons of sediment a year, the study says.

Now, however, some 8,000 dams stretch across the river and its tributaries in a drainage basin that covers 1.8 million square miles and flirts with southern Canada. Over the past 30 years, the two scientists calculate, the average amount of sediment that the river carries a year has been half the pre-dam amount.

Also, two centuries of levee building have sent sediment that once maintained the delta directly into the Gulf of Mexico.

This has led to substantial subsidence (or the sinking of land) in the region. Subsidence also happened as people drained the land for development, allowing the soil to dry out and compact.

Meanwhile, sea-level rise has accelerated. For much of the past 7,000 years, sea levels have risen at rates of less than 1 millimeter a year. But during the 20th century, the rate averaged 1.7 millimeters a year. And since 1993, the rate has increased to some 3 millimeters a year, attributed to the effects of global warming.

And the implications for restoration are profound. Deltas retain soils best when the influx pours out over regions already above sea level. That implies breaching levees above, rather than below, New Orleans. But that also puts a sizable chunk of the region’s population in the way of flooding.

In the end, the study also implies that in many areas, current restoration plans represent at best an early start on a rear-guard action that buys time for an orderly retreat from many of the region’s current coastal areas.